Stuck on no instance for `KnownSymbol`

[magthe]

This isn't so much an issue as a request for help.

I have the following

import Data.ByteString.Lazy (ByteString)
import Network.Wreq qualified as W

class Monad m => MonadHttpClient m where
  mHttpGet :: String -> m (W.Status, ByteString)
  mHttpPost :: W.Postable a => String -> a -> m (W.Status, ByteString)

which I attempt to write two simple tests for. The first, for mHttpGet, works fine:

import Test.HMock
import Test.HMock.TH (makeMockable)
import Test.Tasty
import Test.Tasty.HUnit

makeMockable ''MonadHttpClient

httpGetTest :: TestTree
httpGetTest = testCase "Get Test" $ do
  (s, b) <- runMockT $ do
    expect $ MHttpGet "url" |-> (status200, "")
    mHttpGet "url"
  s @=? status200
  b @=? "c"

The second, for mHttpPost, won't type check, and I don't know how to solve it:

httpPostTest :: TestTree
httpPostTest = testCase "Post Test" $ do
  (s, b) <- runMockT $ do
    expect $ MHttpPost "url" ("hello" :: ByteString) |-> ()
    mHttpPost "url" ("hello" :: ByteString)
  s @=? status200
  b @=? "a"

The errors are

test/HttpTest.hs:44:5: error:
    • No instance for (GHC.TypeLits.KnownSymbol name0)
        arising from a use of ‘expect’
    • In a stmt of a 'do' block:
        expect $ MHttpPost "url" ("hello" :: ByteString) |-> ()
      In the second argument of ‘($)’, namely
        ‘do expect $ MHttpPost "url" ("hello" :: ByteString) |-> ()
            mHttpPost "url" ("hello" :: ByteString)’
      In a stmt of a 'do' block:
        (s, b) <- runMockT
                    $ do expect $ MHttpPost "url" ("hello" :: ByteString) |-> ()
                         mHttpPost "url" ("hello" :: ByteString)
   |
44 |     expect $ MHttpPost "url" ("hello" :: ByteString) |-> ()
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

test/HttpTest.hs:44:14: error:
    • No instance for (Expectable
                         cls0
                         name0
                         IO
                         ()
                         (Action
                            MonadHttpClient
                            "mHttpPost"
                            b0
                            (Network.HTTP.Types.Status.Status, ByteString)))
        arising from a use of ‘|->’
    • In the second argument of ‘($)’, namely
        ‘MHttpPost "url" ("hello" :: ByteString) |-> ()’
      In a stmt of a 'do' block:
        expect $ MHttpPost "url" ("hello" :: ByteString) |-> ()
      In the second argument of ‘($)’, namely
        ‘do expect $ MHttpPost "url" ("hello" :: ByteString) |-> ()
            mHttpPost "url" ("hello" :: ByteString)’
   |
44 |     expect $ MHttpPost "url" ("hello" :: ByteString) |-> ()
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I'm not at all sure what this even means, but I'm guessing it's related to the Postable a constraint in some way.

Any help and/or insights would be very helpful.

[magthe]

I'm using version 0.1.0.1 of the library, and GHC 8.10.4.

[cdsmith]

Ah, yes, that's an ugly error message. It's the second error that is relevant here. In order to use expect $ MHttpPost ..., you need MHttpPost to have an Expectable instance. makeMockable generates such an instance only if mHttpPost is simple enough.

The solution is to expect with a Matcher instead of an Action. That looks something like this: expect $ MHttpPost_ (eq "url") anything |-> (). The anything can be replaced by a more specific predicate, but it must match the rank-2 type forall a. W.Postable a => Predicate a. In other words, it must be able to be interpreted for any argument type. If W.Postable is a subclass of Typeable or you're willing to add Typeable to the signature of mHttpPost, then you could use typed @ByteString (eq "hello") to only match calls where the argument is a ByteString and equal to "hello". In this case, HMock needs to inspect the type of the argument at runtime to only match when it's a ByteString. That's just not possible in Haskell without Typeable.

[magthe]

Switching to a Matcher made it work with anything but in the end I think I'll go for adding the Typeable a as that simplifies writing those predicates quite a bit.

Thanks for the help!

[cdsmith]

Thanks for the question. I've added https://github.com/cdsmith/HMock/blob/main/README.md#why-does-ghc-complain-about-no-instance-for-expectable--action- to the readme file.

[cdsmith]

I have fixed this error message in HEAD, and it'll be much nicer in the next release.

[magthe]

Very nice! I've been pushing on, trying out HMock to replace some of the tests I have where I in the past have used custom monad stacks for testing purposes. So far it's a pleasure to work with.